The present invention relates to a pressure fixing and developing apparatus in which ink containing sheet material to be treated is passed through a high pressure nip defined by a pair of rollers and is also directed to rollers for such apparatus.
It is known to pass sheet media on which an image is to be fixed or developed through a high pressure nip defined by a pair of rollers. Commonly, the outer ends of at least one of the two rollers is loaded with a pressure applying load applied in a direction normal to the longitudinal axis of the roller. This load also biases the rollers together to form the nip.
Application of a load in this manner results in bending moments in the loaded roller. The bending moments cause the roller to deflect or bow in the center such that there is a reduced or minimum fixing or developing pressure at the center of the nip. This deflection near the center of the roller increases when a sheet of media is inserted in the nip. As a result, uneven fixing of toners and ink to the media occurs over the surface area of the sheet. Higher pressures than necessary to fix or fuse toners and ink are required at the ends of the nip to assure adequate pressure fixing at the center of the nip.
There are devices in the prior art in which the rollers are skewed. That is, their longitudinal axes are supported out of parallel with one another, to compensate for the deflection of the fixing rollers. Skewing the rollers allows the ends of the rollers to wrap around each other as they deflect under load, resulting in more uniform pressure along the nip. However, skewing the rollers also results in forces which act on the media in a direction substantially perpendicular to the path the media travels. These lateral forces tend to crease or curl the media during passage through the nip. Also, skewed rollers are generally limited to pressure fixing at one pressure value. For example, higher pressures cause greater deflection in the rollers and require a greater skewing angle to avoid non-uniformity along the nip.
Other expedients have been introduced in an attempt to overcome the problem of deflection of fixing rollers upon the application of force to the ends of the rollers. Large diameter fixing rollers reduce, but do not eliminate, the deflection. However, large diameter fixing rollers add weight, cost and bulk to the apparatus. A backup roller or rollers in pressure contact with the pressure fixing and developing rollers has also been used to urge the pressure fixing rollers together along the nip. However, such backup roller systems require additional space and accessory components in comparison to dual roller systems.
Another method suggested in the prior art to compensate for the deflection due to loading is the use of a roller which is crowned at the center. However, crowned rollers have a faster surface velocity at their center than at their ends. This differential in surface velocity contributes to wrinkling of the media and limits the versatility of these devices in handling various types of substrates.
As a more specific example of the prior art, U.S. Pat. No. 4,363,862 of Giorgini discloses an apparatus for fixing toner powder images on sheet material. In Giorgini, a non-compliant pressure roller and compliant back up roller are supported on skewed longitudinal axes. The pressure roller is constructed from a rigid non-compliant material, such as steel, with an outer layer having an irregular surface comprising a plurality of randomly sized dome projections. Chrome is one example of this surface material. The backup roller is described as having a sheath of a compliant material over a central core. Organic polymeric substances are mentioned as suitable for the sheath, with nylon-66, glass filled nylon, hard rubber and acetal resins being specifically mentioned. The core of this backup roller is described as being of a rigid, non-compliant material.
U.S. Pat. No. 4,768,050 of Beery discloses a pair of pressure rollers used in conjunction with the "Mead" imaging process in which photosensitive micro capsules are ruptured by the rollers to provide the image. This apparatus includes a first roller having a hollow annular shell with an axially central part defining a loading region. A loading shaft extends through the shell and is joined to the shell only at the central loading region. The interior surface of the shell is partially tapered, and is described as controlling the deflection of the shell portion of the roller when the roller is loaded at its ends. The rollers define a working zone and are relieved at their ends to provide non-working regions. With this design, due to the excessive pressures at the ends of the rollers, the working zone must be less than the width of the rollers, as otherwise sheet material passing through the rollers can be damaged. Therefore, this apparatus lacks versatility in being able to effectively treat sheet material of varying widths.
U.S. Pat. No. 4,356,764 of Haugen discloses a pair of rollers, each including a central core of a uniform diameter and an outer hollow shell. Pressure transfer rollers support the shells on their respective cores with the transfer rollers engaging the supported shells at four spaced locations. Because the shell is supported at discrete locations by the transfer rollers, a less than uniform pressure would be expected along the nip. This is particularly expected to be the case when one of the outer cylindrical shells is made of an elastic material, such as rubber of about 35-60 durometer, as mentioned in one embodiment of this apparatus.
The use of pressure fixing rolls for fusing or spreading solidified hot melt ink on print media is also known. Japanese Patent No. 18,351 of Moriguchi, et al. and U.S. Pat. No. 4,745,420 of Gerstenmaier are two examples of these types of devices. The construction of pressure rollers is not described in detail in these two patents. The Gerstenmaier patent teaches an adjustable eccentric for varying the pressure applied to one of the rollers.
U.S. Pat. No. 4,889,761 discloses substrates printed with light-transmissive phase change inks that are processed by application of pressure, or by a combination of heat and pressure, in a dual roller assembly. Rollers having various constructions are disclosed, including a Teflon.RTM.-coated heated roller constructed from aluminum tubing, and a silicone rubber covered pressure roller.
Several prior art references disclose specialized rollers for use in various environments. Canadian Patent No. 712,794, for example, discloses rolls for use in paper machines intended to produce webs of an even thickness and surface contour. The roller apparatus is designed to permit variations in the nip pressure, while maintaining even pressure along the length of the nip. Each roll comprises a rigid core and shell, with an elastic interlayer arranged between the core and the shell. The interlayer may comprise a plurality of layers of compressible rubber having varying elastic properties to permit selective indentation of the outer shell.
U.S. Pat. No. 4,253,392 teaches a roll fuser wherein one of the rolls comprises a thin outer shell having a variable taper producing a concave configuration under high humidity conditions and a generally cylindrical configuration under low humidity conditions. This is accomplished by axially adjusting end plugs supporting end portions of the normally concave roller. This roller design is intended to reduce wrinkling of copy paper under high humidity conditions.
U.S. Pat. No. 4,305,191 teaches an adjustable roller for squeezing liquid from cloth. The squeeze roller includes a support shaft supporting a cylindrical roller having an outer rubber layer. During application of pressure on the rubber cover, the central portion of the roller is subjected to a higher pressure than the opposite ends. Application of fluid pressure to movable support members provides a uniform pressure distribution over the length of the nip. This arrangement thus permits adjustment to vary the pressure along the nip, with the central portion subjected to a higher pressure than the opposite ends.
U.S. Pat. No. 4,372,247 teaches a dual roller fusing apparatus for fixing toner material to a copy sheet. The apparatus comprises a pair of parallel, cold pressure fixing rollers, each of the rollers having an outer cylindrical metal tube, an inner concentric shaft, and a cured elastomer occupying the volume between the tube and the shaft and bonded thereto. Force is applied to one of the two shafts to apply a uniform pressure to the nip.
Other examples of prior art image fixing apparatus including rollers are described in U.S. Pat. No. 3,293,059 of Stowell, U.S. Pat. No. 3,566,076 of Fantuzzo and U.S. Pat. No. 4,568,949 of Muranaka.
Prior art pressure fixing rolls known to the inventor are not satisfactory for treating different types of media having varying thicknesses and/or widths. Application of uniform pressure along the length of the nip is still problematic. Moreover, application of uniform pressure across the width of the substrate (i.e., the length of the nip) may result in wrinkling and dimensional distortion of the substrate.